Effect of initial notch orientation on fracture toughness in fail-safe steel
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NANOSTRUCTURED MATERIALS
Effect of initial notch orientation on fracture toughness in fail-safe steel Tadanobu Inoue • Yuuji Kimura
Received: 13 July 2012 / Accepted: 6 September 2012 / Published online: 6 October 2012 Ó The Author(s) 2012. This article is published with open access at Springerlink.com
Abstract A 0.4C–2Si–1Cr–1Mo steel bar with an ultrafine-elongated grain (UFEG) structures was produced by multi-pass warm caliber rolling. The test sample was machined from the rolled bar with 0°, 45°, and 90° rotation along the rolling direction, and a static three-point bending test was conducted at ambient temperature. The toughness anisotropy on the steel with UFEG structures were studied, including the crack propagation on the basis of the microstructural features. The strength and toughness decreased with an increase in the rotation angle along the rolling direction. The toughness decreased drastically, compared to the strength. The notch orientation dependence on toughness is due to differences in the spatial distribution of weak sites such as {100} cleavage planes and boundaries of elongated grains. For the toughness design in ultrafine-grained materials, it is essential to understand the spatial distribution of these weak sites as well as the grain size.
Introduction Achieving strength and toughness in materials, that is, high strength in combination with resistance to fracture, is one of an eternal issue in the field of structural metallic materials. Since strengthening of materials reduces toughness [1–3], the idea for improving strength–toughness balance is always sought. It is reported that the refinement of crystal grains is an effective method for developing strength and toughness in metallic materials without the addition of T. Inoue (&) Y. Kimura National Institute for Materials Science, 1-2-1, Sengen, Tsukuba 305-0047, Japan e-mail: [email protected]
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alloying elements. However, compared with microstructure evolution through severe plastic deformation (SPD) and strengthening by grain refinement, there have been few studies on their toughness. The study of toughness is far more important in the development of ultrafine-grained (UFG) materials for structural and other applications. Although a reduction in the grain size leads to higher strength on the basis of the Hall–Petch relation, it does not always lead to the improvement of toughness. In recent grain refinement studies, Hohenwarter et al. [4] showed that the ductile-to-brittle transition temperature, DBTT, was higher for submicrocrystalline grain sizes than for microcrystalline microstructures through a compact tension test for UFG armco iron processed by high-pressure torsion (HPT) and subsequent heat treatments. Furthermore, they showed that the fracture toughness depends strongly on crack orientations machined from the HPT disk. Toker et al. [5] indicated the significant effect of anisotropy on the impact toughness of UFG niobium–zirconium alloys processed by equal-channel angular extrusion. We [3, 6, 7] developed stronger, t
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